Smart Parenting During and After Divorce: Introducing Your Child to Your New Partner
The advice in this artilce will be very difficult for most of you to agree with. That being said, let me also say that generalizing about people whose lives may be very complicated is difficult to do, so these are just general guidelines not informed by your particular story.
My rule of thumb is that divorced parents should keep children out of their social lives until they have been separated or divorced for a period of at least two years and you have known your potential new partner for at least a year. Let me explain the easy things first.
It is definitely not a good idea to include your children as participants in social outings with someone you are having an extramarital affair with. This places your children in a very uncomfortable circumstance, and some judges consider this a very egregious example of poor parenting judgment, which might ultimately result in your losing custody of the children. It doesn't matter if "the marriage was over a long time ago" in the figurative sense, either.
Your children might still consider you and your soon-to-be-ex a family, and they may they feel uncomfortable or betrayed by your including them in activities with your boyfriend or girlfriend.
I once worked on a case in which a mother was locking herself in the bedroom and talking for hours via computer with the person she was having an affair with. Because she was not very computer literate, she would ask her son to help her get online. Later this child found out (through the equally poor judgment of his father) that the mother was conducting an online romance. The child believed he somehow participated in the breakup of the marriage because he unwittingly "helped" her talk to her boyfriend. This mother never understood why her son could not adjust to the boyfriend's presence in the house later on.
Do not bring your children into your extramarital relationships. Also, even if it is true (and it rarely is true), no one will believe that the person whose children had playdates with your children, who you wound up having an affair with, was not your boyfriend or girlfriend the whole while.
In the worst of all circumstances, your children are enlisted as "spies" by the parent who suspects the affair and are interrogated mercilessly, or they become "secret agents" who are sworn to secrecy about their parents' affairs. Don't do this to your children; they have enough to worry about.
You might think your new partner is the greatest thing since sliced bread, but at one time you thought the same thing about the person whose name is on the bottom of the restraining order you just got. It's hard to resist the power of someone who not only makes you feel good about yourself but reinforces your negative feelings about your ex.
With all of that conflict to concentrate on (especially if both of you are going through divorces), who has time to create trouble in the new relationship? What happens as a result is an extended "honeymoon period" in the new relationship. Having your kids along with your new partner helps legitimize the relationship, especially if your kids like your new partner's kids and everyone gets along—but it might very well place unnecessary pressure on the kids.